The invention relates to an ultrasound sensor with a polymer foil which is fastened at least at its outer portion to a support body and which is piezoelectrically activated at least in part of the area which is electrically coupled to electrodes.
Shock waves in the ultrasound range are short pulses with components of high frequencies of more than 1 MHz and frequently very high pressures of more than 1 kbar. These shock waves are used, for instance, in lithotripsy for the destruction of concrements, for instance, as kidney stones, as described in greater detail in DE-OS No. 33 28 051. Especially in this medical application it is necessary to measure the shock waves accurately and to monitor the operation of the lithotriptor. To this end, a sensor must be made available which can present the waveform of the shock wave in question with sufficiently large bandwidth. The sensor must furthermore be capable of measuring the wave shape of the shock wave also at the point of the highest pressure, i.e., at the focus where the concrement to be destroyed is placed, in a reproducible manner without being destroyed itself. In other words, it should be possible to measure relatively high pressures of more than 1 kbar reliably, and the sensor itself must be of a rugged design to withstand the high pressures.
At points of lower pressure, diaphragm hydrophones of various designs have heretofore been used as ultrasound sensors (P. A. Lewin, Ultrasonics, September 1981, pages 213 to 216; R. C. Preston et al, J.Phys. E.: Sci. Instrum., vol. 16 (1983), pages 786 to 796). In all cases, the sensor contains as the measuring foil the piezoelectric polymer polyvinylidene fluoride (PVDF) to which metallization is applied.
With PVDF, a large bandwidth of, for instance, 10 MHz can be realized. It has been found, however, that all sensor types of the above-mentioned design are not suitable for detecting shock waves at the focus of a shock wave source, since they are not able to withstand the high pressure at the focus, the amplitude of which can be in the range of about 1 kbar (10.sup.8 Pa) with a steep rising flank of the shock wave, where the rise time can be less than 1 .mu.sec. Ultra sound sensors of the above-mentioned design are ordinarily inoperable after a few shock wave pulses. Especially vulnerable are the electrical contacts in the vicinity of the sensitive point of the sensor, i.e., near the activated region of the polymer foil, which are destroyed when shock waves are detected.
In the European patent application with the publication No. 0 227 985, a sensor of the type mentioned at the outset is proposed which can detect shock waves also at the focus of a lithotriptor in a time-wise stable manner. In this sensor, the metallic electrical contacts are physically separated from the piezoelectrically active zone of the PVDF diaphragm or foil. This arrangement can lead to a relatively low sensitivity.
It is an object of the present invention to provide an ultrasound sensor of the type mentioned at the outset with a piezoelectrically activated polymer foil which can be used particularly for measuring high-energy shock waves with adequate sensitivity and which has a long service life.
According to the invention, the object is achieved by the provision that a first electrode rests against the activated portion of the polymer foil and a second electrode is arranged physically separated from the piezoelectrically activated portion.
Upon the arrival of an ultrasonic shock wave pulse, charges are generated in the piezoelectrically activated region of the polymer foil. The first electrode loosely touches the piezoelectrically activated portion, (i.e., contacts the polymer foil under resilient pretension), for instance, on the back of the polymer foil, which consists particularly of PVDF. The first electrode is connected to the first input of a preamplifier. The second electrode is designed, for instance, as a grid and is connected to ground. The second input of the preamplifier is likewise connected to ground. In this manner, the signal is metallically taken off from the piezo-active point of the polymer foil and is amplified. Due to the design of the invention, sufficiently high sensitivity and long service life of the measuring sensor are achieved, especially for high pressures.
Further advantages and embodiments of the invention will be seen from the detailed description of an exemplary embodiment of the invention, making reference to the figure.